Method for making thin film glass elements



United States Patent 2 Claims. (Cl. 6521) This is a division of mycopending application (now abandoned) Ser. No. 219,045, filed Aug. 23,1962, and assigned to the assignee hereof.

The present invention relates generally to the glass making art and isspecifically concerned with a new method of making glass bodies andparticularly sheets of glass having very high electrical resistance.

It has long been generally recognized that the discovery of a glasshaving electrical resistance approximating that of mica could haveimportant scientific and commercial consequences. Thus, for lack of sucha material, the art has had to compromise between mica, fused silica orquartz and the ordinary silicate or soda-lime glasses for a Wide varietyof uses and purposes. By virtue of my present invention, however, it ispossible to produce bodies of gerrnanate, phosphate and silicate glasseswhich will meet and satisfy requirements which previously compelled theforegoing compromises. It is, for example, possible and feasible throughthis invention to produce a thin film capacitor of silicate, phosphateor germanate glass having superior electrical characteristics coupledwith heat stability and moisture resistance over a wide range oftemperature and a prolonged period of time. The unique workability ofthese glasses permits easy production at relatively low temperatures ofbodies or articles of predetermined sizes and shapes. Thus, while fusedor vitreous quartz must be worked at temperatures of the order of 2000C., the present glasses can readily be worked to form practically anydesired shape at temperatures below 1000 C.

One of my discoveries underlying this invention in all its aspects isthat silica (SiO phosphorous pentoxide (P 0 and germanium dioxide (GeOcan each be compounded with a secondary component in a certain criticalmanner and processed in a particular way to pro duce a glass body havingproperties and characteristics wholly different from silicate, phosphateand germanate glass bodies heretofore known. More specifically, I havefound that contrary to expectations, the electrical resistivity and theviscosity and the hardness of these glasses increase as the content ofthe secondary component is increased. In addition, I have found that thesecondary component must be present in substantial amount, i.e., from 30to 60 mole percent if the new properties and characteristics of theproducts of these materials are to be consistently obtained. A furtherdiscovery is that the secondary component must be an alkaline earthmetal oxide Or mixture thereof and I have also found that calcium oxide,barium oxide, and strontium oxide are particularly efiicacious. Amiscibility discontinuity in portions of these systems imposes thenecessity for a minor quantity of a third component in certain instancesif a homogeneous product is to be obtained. Aluminum oxide, potassiumoxide or one of the other oxides mentioned just above may be employedfor this purpose.

Glasses prepared in accordance with the compositional and processrequirements indicated above and subsequently to be described in detailare capable of being blown or drawn out or pressed or otherwise extendedto produce membrane-like bodies suitable for use, for example, in thinfilm capacitors. Because of viscosity characteristics of glasses of thisinvention under rela- Patented May 16, 1967 tively broad temperatureranges, bubbles may be blown in the course of producing thin glass filmsof the required dimensions for a variety of uses. Sections cut fromlarge bubbles of this glass will normally be of thickness within afairly broad and readily controlled range related to the bubble size andwill be of flatness adequate to a number of different purposes, but inany case may easily be flattened in a warm pressing operation. Thehomogeneity of these glasses is readily estabished in accordance withthe present new process and consequently sections cut from large bubbleswill consistently be substantially uniform throughout in electrical andphysical characteristics.

In accordance with this invention, a polycomponent silicate, phosphateor germanate glass having a roomtemperature electrical resistivity of atleast 10 ohm-cm. is employed. The silicate and phosphate glasses usedhave resistivity values substantially greater than that of fused silicaover the range of temperature from room temperature to more than 500 C.The germanate glasses have resistivity values more than two orders ofmagnitude higher than ordinary GeO glass over the range from 200 C. to450 C. The dielectric constant of this glass at room temperature is 16.5and increases slowly to 17.5 at 320 C., while between room temperatureand 300 C. the loss factor, tan 6, is less than 10- for a frequency of10 kc. The glasses used in this process all have in common an alkalineearth metal oxide component in an amount between 30 and 60 mole percentof the total glass composition.

The method of this invention, generally stated, comprises the steps ofproducing a glass in which the network former is P 0 GeO or SiO and thenetwork modifier is present in the glass in an amount of from 30 to 60mole percent, then forming a sheet of this glass of desired thicknessfrom one-half mil to one-half inch. In a preferred practice, phosphoricacid or finely-divided silica or germanium dioxide and from 30 to 60mole percent of an alkaline earth metal oxide or its carbonateequivalent are mixed together and the resulting substantially dry andhomogeneous mixture is heated to react the glass network former with thecarbonate or oxide to produce a high resistivity glass. Subsequently, abubble of this glass is formed and finally a portion of the glass bubbleis selected and shaped to predetermined form.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth belowtaken in conjunction with the drawings accompanying and forming a partof this specification, in which:

FIG. 1 is a chart bearing curves illustrating electrical resistivitydata obtained in tests of a variety of glasses and including silicateglasses having utility in the present invention;

FIG. 2 is another chart comparing resistivity data obtained for severalsodium-containing glasses of this invention with each other and withvalues for fused silica containing various amounts of sodium; and

FIG. 3 is still another chart bearing curves comparing the resistivityover a range of temperatures of fused Ge0 and a barium oxide-germanateglass useful in this invention.

As indicated above, the materials used in this invention method arepolycomponent glasses in that they contain at least one component inaddition to silicate, phosphate or germanate. Furthermore, the secondarycomponent must be an alkaline earth meta-l oxide. Also, as indicatedabove, this secondary constituent must be present in an amount of from30 to 60 mole percent of the glass body or mass and it must serve as aso-called glass network modifier as distinuished from aso-called glassnetwork former. These glasses thus are composed of a network former inthe form of silicate, phosphate or ermanate and a network modifier whichmay be calcium xide, barium oxide, strontium oxide, magnesium oxide r amixture of two or more of these oxides. Still furner, where it isnecessary to production of a homogeneous ody to overcome theimmiscibility of the network modi- .er in the network former, anadditional component such s aluminum oxide or potassium oxide isemployed as art of the secondary constituent of the composition. Themiscibility promoting additive may be suitably employed u an amountranging from about one mole percent to ive mole percent of the ultimateglass composition and t may be incorporated in the raw mixture of oxides)reparatory to the heating step. Alternatively, it may be tdded when thebasic components of the glass are in the iquid state.

The amount or proportion of the secondary constituent )f these glasscompositions is highly critical to the con- ;i$tent production of glasssheet products having the mique properties of this invention. If thesecondary constituent content of the entire composition is less than 30mole percent, one or another or all of the special desirable propertiesof these products will be significantly impaired or even destroyed. Onthe other hand, if the secondary constituent aggregates more than 60mole percent of the ultimate network former-network modifiercomposition, one or more of these same properties will be againadversely affected to an important extent. It will be understood,however, that other materials may be added to the basic glass consistingof the network former and network modifier components to obtain specialeffects or results. These additives, however, do not constitute thebasis upon which the critical 30-60 mole percent range of the secondaryconstituent is calculated. Further, the use of such additives and theamounts employed will depend upon the specific properties desired in agiven case and the effects which relatively large amounts of optionaladditives have upon the desired physical and electrical properties ofthe glass products.

Bodies of these glasses of various dimensions and shapes may be producedfor a variety of uses. I have, for example, produced membrane-like glassbodies suitable for use as thin film electrical capacitors and I havealso succeeded in producing glass plates, sheets, and strips ofthickness approaching /2 inch. The thickness limitation at the upper endof the range is set by practical considerations while that at the lowerend of the range is fixed by the use to be made of such materials.Because of the close similarity between these new glasses and micas inrespect to their dielectrical strengths, I have produced sheets of theseglasses ranging in thickness from /2 mil to 15 mils to test theirsuitability as a mica-substitute for a variety of electricalapplications. On the basis of these tests, I have been able to establishthat over the full thickness range of micas in general use, these glassproducts can perform the same functions and produce the same results asmicas. However, because of the ease of production of homogeneous bodiesof uniform predetermined thickness and any desired size and the abilityto control the dielectric strengths and other important properties ofthese glass products, they may be preferred to micas for many commercialuses.

The following illustrative, but not limiting, examples of the practiceof this invention method in the preparation of my glass products areoffered in the interest of further apprising those skilled in the art asto representative details and specific data.

Example I Reagent grade calcium carbonate and powdered quartz are mixedtogether in proportions corresponding to 40 mole percent of calciumoxide and 60 mole percent of silica, and then heated in air in aplatinum crucible. Heating continued for one hour with the temperaturebeing maintained at 1600 C. and the mixture therefore in a molten state.The resulting glass has a resistivity of ohm-cm. at room temperature andresistivity over several hundred degrees centigrade as indicated bycurve A of FIG. 1. By way of comparison, curve B of FIG. 1 representsthe resistivity characteristics of fused silica containing less thanone-twentieth as much sodium, while curve C depicts the characteristicsof a soda-lime silicate glass containing 17 weight percent Na O.

Example II In a repetition of the Example I experiment, a mixture ofequal parts of C and SiO was prepared and the resistivity of this glassover the same broad temperature range was determined and plotted ascurve D of FIG. 1.

Example III Curve E of FIG. 1 represents the resistivity of stillanother glass prepared, in general, according to the procedure set forthin Example I. In this case, however, barium carbonate and silicic acid(both of reagent grade) were used in proportions leading to theformation of a 30 mole percent Ba0-70 mole percent SiO glass on heatingin air for 1450 C. for one hour. This glass contained approximately 10times more sodium than did the fused silica of curve B.

Example IV Melting a mixture of 48 mole percent MgO and 52 mole percentSi0 (both reagent grade) at 1600 C. and holding at that temperature foran hour in an air atmosphere, as described in Example I, lead to theformation of a glass, the resistivity characteristics of which areillustrated by curve F of FIG. 1. The sodium content of this glass waslikewise 10 times greater than that of the curve B material.

Example V Using reagent grade MgOO CaCO and SiO (as silica sand),another of these new glasses was produced having theresistivity-temperature properties depicted by curve G of FIG. 1. Thisglass was composed of 50 mole percent SiO- and 25 mole percent of eachCaO and MgO and had a sodium content amounting to about 10 times that ofthe curve B fused silica.

Example VI A mixture of calcium carbonate and phosphoric acid 'wasmelted in air in an open platinum crucible and maintained at 1450C. forone hour. With the resulting mass in the molten state, a stream of drynitrogen (dew point 40 C.) was bubbled through it for 30 minutes todrive off the last traces of moisture and then melt was cast. Analysisof the resulting glass established its composition as 33 mole percentC20 and 67 mole percent P 0 with about 500 parts per million of sodium.The resistivity of this glass at 350 C. is indicated at K on the chartof FIG. 2 where the log of resistivity at 350 C. is plotted againstsodium concentration in ppm. for fused silica (curve L) and for theglass of Example VII (point M) and the glass of Example II of mycopending application, Ser. No. 128,447 (point N), and a calciumsilicate glass of this invention having the composition CaO.-1.5Si0(point P).

Example VII resistivity of this glass at room temperature is greaterthan 10 ohm-cm.

Example VIII A germanate glass was made according to the foregoingprocedure by mixing barium carbonate and germanium dioxide powders andmelting the resulting mixture in a platinum crucible in air andmaintaining its temperature at 1300 C. for one hour and then casting andcooling the mass. This glass had a composition approximating 39 molepercent BaO and 61 mole percent GeO and an electrical resistivitycharacteristic illustrated by curve R of FIG. 3. Curve S represents theelectrical resistivity of Ge0 (fused) over the same temperature rangeand it is seen that while the impurity levels of these two materials arethe same (reagent grade GeO and barium carbonate being used throughout),the resistivity of the new glass is more than two orders of magnitudegreater than that of the G602 over the range from 200 C. to 450 C. andthe extrapolated resistivity of the glass at room temperature is greaterthan ohm-cm. The dielectric constant of this glass at room temperatureis 16.5 and increases slowly to 17.5 at 320 C. while the loss factor,tan 5, is less than 10 for a frequency of 10 kc. As in the priorexamples, the specimens were prepared for test and tested in accordancewith the procedures described in said copending application, Ser. No.128,447.

As generally indicated above, products of these glasses may containsubstances in addition to the glass network former and the glass networkmodifier. Thus, in the case of magnesium silicates, magnesium phosphatesand magnesium germanates, it is essential that an amount of amiscibility promoting agent be present. This is the case also forcertain calcium silicate, phosphate and germanate glasses of thisinvention, specifically those containing less than 27 mole percent ofCaO. In any event, however, from one mole percent to five mole percentof this agent is sufiicient for this purpose and the use of asubstantially larger amount can impair the desired properties of thesenew products. The use of this agent is necessary when the amount of theglass network modifier is less than the amount stated below:

Other substances such as A1 0 may also be present in readily detectableamounts in these glasses without materially impairing the properties ofproducts made from them. Preferably, however, the amount of inertimpurities will not exceed five mole percent of the glass composition.

These glasses, as the data represented by the accompanying chartsillustrate, have in common the important property of alkali metal ionimmobility. The strong tendency for these ions to move in response tothe application of an electrical potential is olfset by the alkalineearth ions of these glasses and since the alkaline earth ions themselvesdo not move under such condition, these glasses have high resistivityvalues, there being no effective current-conducting means within theglass bodies or masses.

In will be understood that throughout this specification and in theappended claims, Whatever amounts, proportions, ratios or percentagesare stated, reference is made to the weight basis rather than to thevolume basis, and mole percent is employed for convenience to expressweight relationships of reagents and constituents.

Having thus described this invention in such full, clear, concise andexact terms as to enable any person skilled in the art to which itpertains to make and use the same, and having set forth the best modecontemplated of carrying out this invention, I state that the subjectmatter which I regard as being my invention is particularly pointed outand distinctly claimed in what is claimed, it being understood thatequivalents or modifications of, or substitutions for, parts of thespecifically described embodiments of the invention may be made withoutdeparting from the scope of the invention as set forth in what isclaimed.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In the method of making a glass sheet having special utility inelectrical insulating applications including the steps of forming aglass bubble and then shaping a portion of the glass bubble topredetermined form and thickness between one-half mil and one-half inch,the combination of the preliminary step of providing as the glass to beused in the formation or" the glass bubble a glass havingroom-temperature resistivity of at least 10- ohm centimeters consistingof about 40 mole percent calcium oxide and about 60 mole percent silicaand containing sodium in an amount at least 10 times greater than thesodium content of fused silica having resistivity less than that of saidglass over the temperature range of 300 to 500 C.

2. In the method of making a glass sheet having special utility inelectrical insulating applications including the step of forming a glassbody of desired shape and dimensions, the combination of the preliminarystep of providing as the glass to be used in the formation of the saidbody a glass having room-temperature resistivity of at least 10- ohmcentimeters consisting of from 30 to 60 mole percent of an oxideselected from the group consisting of calcium oxide, barium oxide,strontium oxide, magnesium oxide and mixtures thereof, and from 70 to 40mole percent, respectively, of an oxide selected from the groupconsisting of P 0 Ge0 and SiO and contain-ing sodium in an amount atleast 10 times greater than the sodium content of fused silica havingresistivity less than that of said glass over the temperature range of300 to 550 C.

References Cited by the Examiner UNITED STATES PATENTS 2,457,785 12/1948Slayter et al. --187 2,580,662 1/1952 Danzin.

2,780,889 2/1957 Fulk 65-21 2,910,805 11/1959 Muller et a1 65-187 OTHERREFERENCES Ceramic Age, May 1949, pages 260 to 265, entitled Invitationto Glass Technology, by Alexis G. Pincus.

DONALL N. SYLVESTER, Primary Examiner. F. W. MIGA, Assistant Examiner.

1. IN THE METHOD OF MAKING A GLAS SHEET HAVING SPECIAL UTILITY INELECTRICAL INSULATING APPLICATIONS INCLUDING THE STEPS OF FORMING AGLASS BUBBLE AND THEN SHAPING A PORTION OF THE GLASS BUBBLE TOPREDETERMINED FORM AND THICKNESS BETWEEN ONE-HALF MIL AND ON-HALF INCH,THE COMBINATION OF THE PRELIMINARY STEP OF PROVIDING AS THE GLASS TO BEUSED IN THE FORMATION OF THE GLASS BUBBLE A GLASS HAVING ROOM-TEMPRATURERESISIVITY OF AT LEAST 10**-18 OHM CENTMETERS CONSISTING OF ABOUT 40MOLE PERCENT CALCIUM OXIDE AND ABOUT 60 MOLE PERCENT SILICA ANDCONTAINING SODIUM IN AN AMOUNT AT LEAST 10 TIMES GREATER THAN THE SODIUMCONTENT OF FUSED SILICA HAVING RESISTIVITY LESS THAN THAT OF SAID GLASSOVER THE TEMPERATURE RANGE OF 300 TO 500*C.